Fuel cells are devices that convert fluid streams containing a fuel (for example, hydrogen) and an oxidizing species (for example, oxygen or air) to electricity, heat, and reaction products. Such devices comprise an anode, where the fuel is provided, a cathode, where the oxidizing species is provided, and an electrolyte separating the two. As used herein, the term “catalyst coated membrane” means a combination of an electrolyte and at least one electrode. The fuel and oxidant are typically liquid or gaseous materials. The electrolyte is an electronic insulator that separates the fuel and oxidant. It provides an ionic pathway for the ions to move between the anode, where the ions are produced by reaction of the fuel, to the cathode, where they are used to produce the product. The electrons produced during formation of the ions are used in an external circuit, thus producing electricity. As used herein, fuel cells may include a single cell comprising only one anode, one cathode and an electrolyte interposed therebetween, or multiple cells assembled in a stack. In the latter case there are multiple separate anode and cathode areas wherein each anode and cathode area is separated by an electrolyte. The individual anode and cathode areas in such a stack are each fed fuel and oxidant, respectively, and may be connected in any combination of series or parallel external connections to provide power.
Additional components in a single cell or in a fuel cell stack may optionally include means to distribute the reactants across the anode and cathode, including, but not limited to porous gas diffusion media. Various sealing materials used to prohibit mixing of the various species may also be used. As used herein, the membrane electrode assembly (MEA) comprises the catalyst coated membrane and such gas diffusion media and sealing materials. Additionally, so-called bipolar plates, which are plates with channels to distribute the reactant may also be present.
A Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a type of fuel cell where the electrolyte is a polymer electrolyte. Other types of fuel cells include Solid Oxide Fuel Cells (SOFC), Molten Carbonate Fuel Cells (MCFC), Phosphoric Acid Fuel Cells (PAFC), etc. As with any electrochemical device that operates using fluid reactants, unique challenges exist for achieving both high performance and long operating times. In order to achieve high performance it is necessary to reduce the electrical and ionic resistance of components within the device. Recent advances in the polymer electrolyte membranes have enabled significant improvements in the power density of PEMFCs. Steady progress has been made in various other aspects including lowering Pt loading, extending membrane life, and achieving high performance at different operating conditions. However, many technical challenges are still ahead. One of them is for the membrane electrode assembly to meet the lifetime requirements for various potential applications. These range from hundreds of hours for portable applications to 5,000 hours or longer for automotive applications to 40,000 hours or longer in stationary applications.
Although all of the materials in the fuel cell may be subject to degradation during operation, the integrity and health of the membrane is particularly important. Should the membrane degrade during fuel cell operation, it tends to become thinner and weaker, thus making it more likely to develop holes or tears. Should this occur, the oxidizing gas and fuel may mix internally potentially leading to internal reaction. Because such an internal reaction may ultimately cause damage to the entire system, the fuel cell must be shut down. One well known approach to assessing the health of fluorinated membranes is to measure the amount of fluoride ions in the product water of the fuel cell. Higher values of this so-called fluoride release rate are indicative of more attack of the membrane, and therefore are associated with membranes that have lower durability. Correspondingly, lower fluoride release rates are indicative of a healthier membrane, one more likely to have longer life.
As is well known in the art, decreasing the thickness of the polymer electrolyte membrane can reduce the membrane ionic resistance, thus increasing fuel cell power density. However, reducing the membranes physical thickness can increase the susceptibility to damage from other device components leading to shorter cell lifetimes. Various improvements have been developed to mitigate this problem. For example, U.S. Pat. No. RE 37,307, U.S. Pat. No. RE37,701, U.S. Application No. 2004/0045814 to Bahar et al., and U.S. Pat. No. 6,613,203 to Hobson, et. al, show that a polymer electrolyte membrane reinforced with a fully impregnated macroporous membrane has advantageous mechanical properties. Although this approach is successful in improving cell performance and increasing lifetimes, it does not address mechanisms involving chemical attack of the membrane by highly oxidizing species present during fuel cell operation. These include, for example, various radical species such as peroxide and hydroxide radicals that can attack and degrade the ionomer. Thus, the mechanical reinforcement in '307 and the like is a necessary, but generally not totally sufficient, condition for longer life.
The performance of a fuel cell over time is known as fuel cell durability, or fuel cell stability. During normal operation of a fuel cell, the power density typically decreases as the operation time increases. This decrease, described by practitioners as voltage decay, is not desirable because less useful work is obtained as the cell ages during use. Ultimately, the cell or stack will eventually produce so little power that it is no longer useful at all. Furthermore, during operation the amount of fuel, for example, hydrogen, that crosses over from the fuel side to the oxidizing side of the cell will increase as the health of the membrane deteriorates. Hydrogen cross-over is thus used as one measure of membrane life.
A life test is generally performed under a given set of operating conditions for a fixed period of time. The test is performed under a known temperature, relative humidity, flow rate and pressure of inlet gases. In the present application, life tests are performed under open circuit conditions because these are known in the art to give the most accelerated membrane degradation. Thus, if a membrane has limited or no degradation during an open circuit voltage hold, it can be expected to last a much longer time when used in an actual fuel cell under load.
As mentioned above, hydrogen cross-over and fluoride release rate are typically used to determine the extent of degradation, and thereby life, of a fuel cell. For hydrogen cross-over, the amount of hydrogen that crosses over from one side of the membrane to the other is measured after at various times during a life-test. If the hydrogen cross-over is above some predetermined level, 2.5 cm3 H2/min is used herein, then the test is ended, and the life is calculated as the number of hours the cell has operated. Fluoride release rate (FRR) measures degradation products that leave the cell in the product water during a life test. For fluorocarbon membranes, the amount of fluoride ions in the water can be measured, and the rate of production of them is calculated as a fluoride release rate. The lower this number, the less degradation and therefore the longer the membrane will survive, at least assuming the degradation is uniform in the membrane. (Specific details of the test protocol used herein for life determination are described below).
Although there have been many improvements to fuel cells in an effort to improve life of fuel cells, there continues to be an unmet need for even more durable fuel cells, and in particular, more durable membrane materials for use in PEMFCs.